This is a Phase III randomized, placebo-controlled, double-blind, multinational study comparing recombinant antibody (rhuMAbHER2) plus cyclophosphamide and doxorubicin with placebo plus cyclophosphamide and doxorubicin. Approximately 450 patients with HER2/neu overexpression who have not received prior cytotoxic chemotherapy for metastatic breast cancer will be enrolled in the study. Patients will be randomized to one of two treatment groups. Patients will be monitored over time to evaluate the efficacy and safety of rhuMAbHER2.